Liquid crystal displays (LCDs) are typically comprised of two flat glass sheets that encapsulate a thin layer of liquid crystal material. An array of transparent thin film electrodes on the glass modulate the light transmission properties of the liquid crystal material, thereby creating the image. By incorporating an active device such as a diode or thin film transistor (TFT) at each pixel, high contrast and response speed can be achieved to produce high resolution video displays. Such flat panel displays, commonly referred to as active matrix LCDs (AMLCD), have become the predominant technology for high performance displays such as notebook computers and portable televisions.
At present, most AMLCDs utilize amorphous silicon (a-Si) processes which have a maximum process temperature of 450.degree. C. Nevertheless, it has long been recognized that the use of polycrystalline silicon (poly-Si) offers certain advantages over a-Si. Poly-Si has a much higher drive current and electron mobility, thereby allowing reduction of TFT size and at the same time increasing the response speed of the pixels. Poly-Si processing also enables the manufacture of display drive circuitry directly onto the glass substrate (on-board logic). Such integration significantly decreases costs and increases reliability and also allows for smaller packages. By contrast, a-Si requires discrete driver chips that must be attached to the display periphery using integrated circuit packaging techniques such as tape carrier bonding.
Poly-Si is conventionally made by depositing amorphous silicon onto a glass sheet using chemical vapor deposition (CVD) techniques, and subsequently exposing the coated glass to high temperatures for a period of time that is sufficient to crystallize the a-Si to poly-Si. There are many methods for fabricating poly-Si, which can be grouped in two categories: low-temperature poly-Si methods, which utilize processing temperatures up to about 600.degree. C., and high-temperature poly-Si methods, which typically employ temperatures as high as around 900.degree. C.
Many of the low-temperature methods need to employ special techniques to enable crystallization of a-Si to poly-Si. One such technique is laser recrystallization, in which the substrate is held at a temperature of 400.degree. C. and an excimer laser is used to locally melt and recrystallize the Si layer. The main disadvantage of laser recrystallization is the difficulty in achieving good uniformity across the sample. Most of the poly-Si TFTs made by this technique have more than sufficient mobilities for on-board logic, but the fact that only a small area can be melted and recrystallized at any one time leads to uniformity (e.g., stitching) problems. Low temperature poly-Si TFTs can also be made by thermally crystallizing amorphous silicon (maximum temperatures of 600.degree. C.), but in order to make high quality transistors at such low temperatures the films must typically be treated for extended periods of time (e.g. 25 hours or more). In contrast, high temperature processing only requires relatively short process times, and offers the advantage of using other thermal process steps in the manufacture of poly-Si TFT's, such as in situ growing and annealing of the gate oxide, and dopant activation.
The highest quality poly-Si TFTs are fabricated at temperatures of at least 900.degree. C.: such processes enable formation of poly-Si films having extremely high electron mobility (for rapid switching) and excellent TFT uniformity across large areas. This fabrication process typically consists of successive deposition and patterning of thin films using elevated temperature processes which result in the substrate being heated to temperatures in the range of 900.degree. C. Display substrates must not only be capable of surviving such high temperatures without opacifying or warping, but they must also maintain precise dimensional stability. Because the TFT fabrication requires multiple photolithography steps, any irreversible dimensional changes (shrinkage) in the substrate can result in pattern misalignment between successive exposure steps. Permissible substrate shrinkage during display processing depends upon the nature of the circuitry design and the size of the display, and for AMLCDs the shrinkage must amount to no more than a fraction of the smallest feature across the maximum dimension of the display. This can be as small as 5-20 ppm, or a shrinkage of no more than 2.5 to 10 microns over a substrate length of 500 mm.
There are very few materials capable of meeting the thermal stability requirements necessary for high temperature (900.degree. C.) poly-Si processing. One approach has been to use fused silica as the substrate. Fused silica has a sufficiently high strain point of 990.degree.-1000.degree. C. and exhibits very little shrinkage (&lt;50 ppm) when exposed to high temperature poly-Si processes of 900.degree. C. for 6 hours. The thermal expansion of fused silica is significantly lower than that of silicon, however, with a coefficient of thermal expansion (C.T.E.) of 5.times.10.sup.-7 /.degree. C. versus silicon's 37.times.10.sup.-7 /.degree. C. This mismatch can result in a stressed Si film. Furthermore, fused silica substrates are extremely expensive to produce in large size, to the point where using them in large display applications is cost prohibitive.
It would therefore be desirable to develop alternative, less expensive substrate materials which are capable of surviving exposure to the high-temperature poly-Si processes without considerable shrinkage.